Regulations in force require that the capacity of such batteries be periodically verified. This verification is performed by continuously discharging the battery through a rheostat connected across its terminals in order to keep the discharge current to a constant value.
As soon as the voltage across any individual cell drops below a given threshold, that cell is disconnected from the battery and is no longer discharged. Discharging continues for the rest of the battery, with the current being kept to the same constant value.
Each time a cell is disconnected, the time it took to discharge is noted. Given that both the time to discharge and the constant discharge current are known for each cell, it is possible to calculate the quantity of electricity given back by each cell, ie. its capacity.
However, the actual procedure is tedious and dangerous. It is tedious since the operator must continuously measure the cell voltages using a voltmeter throughout the entire period of discharge. A discharge cycle lasts for a long time (about 5 to 10 hours), and the number of cells to be individually tested is high (for example, commonly used batteries in the electricity power supply industry are 48 volt batteries and 127 volt batteries, comprising 24 and 58 cells respectively).
The procedure is also dangerous in that the operator is continually working on apparatus having a dangerous voltage across its end terminals (eg. 48 or 127 volts) and must thus perform the test measurements while wearing protective gloves.
Preferred embodiments of the present invention avoid the need for manual test measurements by warning an operator each time an individual cell has been discharged, ie. that the voltage across its terminals has dropped below a predetermined threshold. The operator may then stop discharging. Apparatus in accordance with the invention may also indicate which cell is discharged together with the time elapsed since discharging began. The operator can then readily disconnect the cell in question, note the corresponding discharge time, and cause discharging to continue until the apparatus detects that another cell has been discharged.
The cycle continues in this manner until the last battery cell has been discharged.